It is common to use centrifugal fans secured to the external structures of buildings for delivering air through ducts to the interiors of the buildings. In a previous application of Applicant's, No. PM 2487 dated 18 Nov. 1993, there was a description of fan closure means which is arranged to be inserted in the eye of a fan case volute, for example at the end of a season, to inhibit down drafts of air from external winds into a building and to inhibit warm air escaping from the building through the duct. Such an arrangement functions very satisfactorily, but requires manual positioning which is sometimes most inconvenient.
Both counterweighted closure flaps and retaining latches for closure flaps are already known in the art, for example in Australian Patent Application Nos. 69233/81 (542813) and 25236/77. However, what is not otherwise known to the Applicant is a combination wherein a resilient latch is associated with a counterweighted flap, but is releasable only when sufficient static pressure is generated by the fan, to allow the flap to open. This is a very desirable feature, since opening can be completely automatic, but the flap is unlikely to "flutter" under windy conditions.